It is often necessary or advantageous to attach a host computer to one or more peripheral devices. Commonly used peripheral devices would include data storage devices, such as disk drives or tape drives. The peripheral device is usually interfaced by means of a bus system. A variety of bus systems are known to the art, including the Storage Module Device (SMD), Enhanced Small Disk Interface (ESDI), ST506 and Small Computer Interface (SCSI). To take one example, the SCSI bus would allow high speed parallel transfer of various types of information, including commands, messages, status and data, from a disk drive with a SCSI interface. The parameters of the SCSI computer bus are defined by agreed upon industry standards, such as ANSI Committee Standard X3.131 1986, and the SCSI-2 standard defined in ANSI working draft X3T9.2/86-109. Devices attached to a SCSI bus are said to operate in the initiator mode if they are the host computer or system, and to operate in the target mode if they are the peripheral device.
Direct attachment of one or more data storage devices to a host computer has typically required that the host computer include a controller to instruct the peripheral device. In the case of attachment of one or more SCSI data storage devices the host computer is required to contain a SCSI controller, sometimes referred to as a SCSI host adapter. In practice, the physical attachment of a number of SCSI data storage devices to a host computer over a SCSI bus consists of one or more wide cables running between the host computer and the SCSI data storage devices. Typically, a maximum of seven SCSI data storage devices may be attached to a host computer with a single SCSI bus.
Testing of peripherals is one known application of networking more than one peripheral device to a host computer. Again with respect to SCSI data storage peripherals, there exist a large number of computer based SCSI analyzers or testers on the market. All of these testers require the use of an SCSI host adapter to attach SCSI devices to the host computer. Further, the computer software to operate SCSI analyzers resides entirely in the host computer which contains the SCSI host adapter. One direct result of this is that the computer software for such SCSI analyzers must maintain a separate control structure for each of the SCSI devices being analyzed. Maintaining such multiple control structures is disadvantageous, in part because it is a complicated and demanding task.
Yet another disadvantage of the currently available systems is the complexity of the networking software residing in the host computer. This arises primarily because the host computer maintains communications with more than one network device at a time by means of interleaving multiple communications onto the network. This interleaving of communications places an additional burden on the host computer.
Yet a further disadvantage of the prior systems is their limited number of devices which can be economically tested with one host computer.